ServletContext傳遞值爲Null解決方法
Web容器在啓動的時候,它會爲每一個Web程序都創建一個對應的ServletContext對象,它代表了當前的Web應用。
作用:
1.共享數據。
我在此Servlet中保存的數據,在其他Servlet中也可以使用。
存放數據文件:
package com.edwin.servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
* @author Edwin D
* @date 2020.6.2 上午 7:59
*/
public class HelloServlet_2 extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// this.getInitParameter();初始化參數
// this.getServletContext();Servlet配置
// this.getServletContext();Servlet上下文
ServletContext i = this.getServletContext();
String username = "Edwin";//數據
i.setAttribute("Username",username);
// 將一個值存在了ServletContext中,名字爲:Username,值爲username(Edwin)。
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
讀取數據文件:
package com.edwin.servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
* @author Edwin D
* @date 2020.6.2 上午 8:56
*/
public class GetServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext i = this.getServletContext();
String username = (String) i.getAttribute("Username");
resp.setContentType("text/html");
resp.setCharacterEncoding("UTF-8");
resp.getWriter().println("姓名:"+username);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
XML文件:
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0"
metadata-complete="true">
<servlet>
<servlet-name>Hello</servlet-name>
<servlet-class>com.edwin.servlet.HelloServlet_2</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Hello</servlet-name>
<url-pattern>/Hello</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>Get</servlet-name>
<servlet-class>com.edwin.servlet.GetServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Get</servlet-name>
<url-pattern>/Get</url-pattern>
</servlet-mapping>
</web-app>
測試結果:
注意訪問頁面的順序:點擊運行,先訪問Hello(此頁面爲空),但是必須要先訪問,確保:
String username = "Edwin";//數據
數據傳入ServletContext,纔可以被後面的GetServlet訪問到。
結果:
主界面:
直接訪問GetServlet:Null值
轉到Hello:空值
再訪問GetServlet:
參考文獻
《【狂神說Java】JavaWeb入門到實戰》
2020.06.02